In analyzing the timing of a diesel engine, determination is made of the number of degrees between the time a piston in a selected cylinder reaches its so-called "top dead center" position and the time that combustion takes place in that cylinder. Automobile manufacturers commonly specify the number of degrees at a specific engine speed as a compromise to numerous considerations which must be taken into account, such as reducing emissions, maximizing gas mileage, optimizing performance, etc. For example, an automobile manufacturer may specify that combustion should start five degrees after the piston has reached top dead center at an engine speed of 1,250 rpm.
Thus, in a diesel timing apparatus, the events of top dead center and combustion must be determined. The top dead center event is identified in the following manner. A diesel engine includes a wheel mounted on the end of the engine crank shaft so as to rotate therewith. A notch or other marking means is located in the periphery of the wheel. A receptacle for a sensing device is located on the engine block and is so located that the notch on the rotating wheel will pass the receptacle a known number of degrees of crank shaft rotation after the number one cylinder has reached its top-dead-center position. In presently manufactured diesel engines, such known number is either 20.degree. or 9.5.degree.. The sensing device may be magnetic in nature and have a transducer to provide an electrical signal 9.5.degree. or 20.degree. after top dead center.
The combustion event may be sensed by light occurring during combustion. In a diesel engine, fuel is first delivered to a prechamber. To increase the temperature of the prechamber to a value to insure engine starting, a so-called "glow plug" is threaded into an access port of each such prechamber. The glow plug is energized a few seconds before starting, but while the engine is actually operating, the glow plug is not energized. It has previously been proposed that while timing a diesel engine, the glow plug for the prechamber associated with the number one cylinder is removed and a luminosity probe inserted in its place. The luminosity probe includes a sensor which responds to light produced during the combustion event and generates an electrical signal.
An electrical signal produced in response to the top dead center event and an electrical signal in response to the combustion event are then processed and applied to circuitry which determines the time between the two. The time is translated to degrees of rotation which are displayed in some convenient manner. The serviceman timing a diesel engine can adjust the engine in order that the displayed angle matches the angle specified by the manufacturer for a particular speed.
It is important that the electrical signal for one event always occur before the electrical signal for the other event. For example, suppose the electrical signal in response to the combustion event occurs first, and the electrical signal in response to top dead center occurs next (the result of the electrical signal produced in response to top dead center being delayed by 9.5.degree. or 20.degree. from the actual top dead center event). Then, it is important that the electrical signal produced in response to the combustion event always occurs first. When the offset is 20.degree., that criterion presents no difficulty because variations in engine performance will rarely if ever be so far off that the electrical signal in response to the top dead center event would occur first. However, when an engine having a 9.5.degree. offset is evaluated, the timing could be such that the electrical signal resulting from a top dead center event could occur before combustion in the corresponding cylinder.